Polycyclic aromatic hydrocarbons: heavy testing

Complex and heavy

Complex heavy fuel oil mixtures can be analysed for their polycyclic aromatic hydrocarbon content using multivariate curve resolution and alternating least squares on their chromatographic and mass spectrometric data, according to research published in Analytical Chemistry.

During the last ten years or so, the technique of choice for obtaining high resolution and high peak capacity has emerged as comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometer (GC×GC-TOFMS). Unfortunately, despite its prowess in separating the components of complex mixtures changes in instrumental conditions (fluctuations in temperature and pressure) and matrix effects as well as stationery phase degradation and various other factors mean that total separations are not necessarily possible. The application of suggested chemometrics analysis to the data have not yet given users the perfect solution. Now, researchers from Iran and Spain have found that multivariate curve resolution alternating least-squares can be used to resolve and quantify PAHs in even the most complex of mixtures, heavy fuel oil samples.

Oil boost

Researchers in the Department of Chemistry, at Sharif University of Technology, in Tehran, Iran working with colleagues in the Department of Environmental Chemistry, at IDAEA-CSIC, in Barcelona, Spain, have improved significantly on results obtained from GCxGC-TOFMS using their analytical approach.

As supplies of oil are gradually dwindling, exploratory teams are constantly on the look out for non-conventional sources, such as heavier oils and shales, and to fully exploit offshore drilling sites. Unfortunately, the pressure for deeper drilling of such offshore sites means that spills and leaks, are inevitable. Such incidents may not be comparable to the recent high-profile incident in the Gulf of Mexico, but spills of around 1000 tonnes are rather common, the researchers say. Remote-sensing technology can be used in the detection of such spills but full characterisation of the materials spilled is necessary to find the culpable source as well as to determine the fate of the spilled material and its likely environmental impact. The team adds that the diverse nature of refinery components used in blending heavy fuel oils means that the variable mix of relatively high molecular weight compounds is difficult to separate and analyse.

"There is still the need for the development of simpler and more reliable methods of multivariate resolution of co-eluted peaks in GC×GC?TOFMS analysis, which should consider their specific chromatographic nature," the team asserts. The potential of the MCR-ALS method has already been demonstrated in other circumstances. The team has now tested a strategy based on column-wise superaugmentation of different slices taken from the first column in different chromatographic runs to which MCR-ALS is then applied to allow them to separate the PAHs with much greater resolution.

Proof is in the separation

Indeed, as a proof of principle, the team was able to discern ten specific polycyclic aromatic hydrocarbons in the aromatic fraction of a heavy fuel oil sample even in the presence of interferences. The researchers describe their method as a "powerful and fast method" for multicomponent resolution, identification and quantification of the target compounds. The team adds that the approach has numerous advantages over the alternatives, PARAFAC (Parallel Factor Analysis) chemometrics analysis, which relies on avoiding time shifts in the two-dimensional gas chromatography.

"The adoption of the proposed methods will strongly depend on instrumentation companies and sofware developers," team leader Romà Tauler told SpectroscopyNOW. "Most people working with chromatography only try to do what is delivered with the instrumentation software." He adds that chromatographic instrumentation companies are not necessarily interested in providing chemometric tools for more sophisticated analyses with less expensive instrumentation.